One aspect of this invention relates to electrical machines which convert mechanical energy into electrical energy, or vice versa, by an interaction between a magnetic field and an electric current. Examples of such machines are electric motors, dynamos and alternators. A further aspect of the invention relates to rotors for such machines.
In a dynamo having a field coil to produce the dynamo's magnetic field, it is well known to vary the electrical output of the dynamo by varying the field current. This way of varying the electrical output of an electrical machine is not possible, however, in the case of a machine whose magnetic field is provided by one or more permanent magnets.